Why can't a knight be a nonbeliever? There may be repercussions for being a heretic, but a heretic that keeps it to themselves (until they don't have to) just makes for a good story.
Also: Merlin was totes not a believer. Nor were, I suspect, Morgana Le Fay nor her bastard son Mordred.
If, of course, your noble knights castigate and ultimately execute heretics, I suspect you're actually running an evil campaign and just don't know it.
Ronin.
Fundamentalist.
Merchant Marine.
Graduate of Miskatonic University.
If you want a game to be about a governing set of values, it's on you to ask if the players are in to that idea. Part of the task of matching the game to the players is on you - and forcing it on them leads to threads like this. If you don't have 100% buy in, don't do it. If you march in to session zero with a "my way or the highway" approach, you'll probably get some people that pay lipservice to the idea, but subvert it later. That's probably on you.
Might as well ban chaotic alignments, while you are at it.